This invention relates to a method and system of processing image signals in which an image area of a lithographic printing plate of an offset printing press or the like is measured, the image area is divided into a plurality of zones, and an image signal is supplied to an image area measuring device that determines the quantity of ink supplied to each zone at the time of printing.
A typical example of such a device is disclosed in U.S. Pat. No. 3,958,509 in which the image area is determined by photographing an image bearing surface with an image pick up tube of a television camera to produce an image signal and then arithmetically processing the image signal.
With this device, however, it is essential to provide a solid portion corresponding to 100% dot ratio in a margin of a lithographic printing plate for the purpose of determining the dot ratio percentage of various portions of the image. Accordingly, it is necessary to provide a solid portion for effecting a proof for a lithographic printing plate. Accordingly, for a lithographic printing plate not provided with a solid portion for the proof, the measurement of the image area cannot be made.
Moreover, when photographing the image bearing surface, there are such problems as nonuniformity in the light quantity irradiated, and decreases in the light quantity near a photographing lens so that it is necessary to remove nonuniformity in the photographing conditions caused by these problems. According to the prior art method, the proof has been made by arithmetic operations including multiplying and subtraction operations so that the time required for the arithmetic operations is large, thus, making it impossible to rapidly determine the image area.